Effect of split-focus approach on producing larger coagulation in swine liver.

The split-focus approach has the potential to substantially improve the throughput of coagulation high-intensity focused ultrasound (HIFU) treatment. A prototype split-focus transducer with four elements at 4.25 MHz, combined with a small imaging probe at 6.5 MHz, was constructed for transrectal treatment of a prostate. Computer simulation predicted that 1.57 times larger acoustic power than a single-spot focus was required for a split focus to heat the tissue up to the same peak temperature at 4 s from the start of insonation. When the peak temperature was 100 degrees C, the tissue volume above 70 degrees C was predicted to be 2.3 times larger for a split focus. Swine liver lobes were intraoperatively insonated with the split-focus transducer for 4 s. A lesion of coagulation necrosis 3 to 4 times larger in volume than with a single-spot focus was formed with a split focus at the same acoustic power ratio as described above. Furthermore, a lesion about 2 times larger in volume was formed when compared at the same acoustic power. These results show the great advantage of a split-focus approach over a conventional single-spot focus in coagulation treatment.